Heater for the maintenance of stable two-phase flow in uninsulated, flexible-transfer-line cryogenic systems



E. ELOVIC ET AL 3,332,254

NANCE OF STABLE TWO'PHASE FLOW IN FLEXIBLE-TRANSFERLINE CRYOGENIC SYSTEMS July 25, 1967 HEATER FOR THE MAINTE I UNINSULATED Filed Aug. 16, 1965 2 Sheets-Sheet 2 REQUIRED M/N/MUM HEATER EX/T QUHL/TY TRANSPORT TUBE- tlwqw Exw QEQMI c Wm m 0 00 E TLD N N 2 E 0 vww T m E 5 T M United States Patent 3,332,254 HEATER FOR THE MAINTENANCE OF STABLE TWO-PHASE FLOW IN UNINSULATED, FLEXI- BLE-TRANSFER-LINE CRYOGENIC SYSTEMS Ernest Elovic, Cincinnati, Ohio, and Andrew I. Dahl, Schenectady, N.Y., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Aug. 16, 1965, Ser. No. 480,213 1 Claim. (Cl. 62-514) ABSTRACT OF THE DISCLOSURE A heater is included in the uninsulated, flexible-transfer-line cooling system of a cryogenic system, the heater being inserted at the exhaust port of the supply dewar to extend the cooling capability of the system.

tubing, through which the coolant is transferred from the supply dewar to the cell dewar, and the necessary pressure and flow control valves.

The successful operation of an uninsulated, flexibletransfer-line cryogenic cooling system depends on the maintenance of stable two-phase flow in which the liquid phase, in the form of drops, is transferred through the transfer line by the vapor phase. The flow regime must be such that the vapor forms a superheated insulating layer of vapor around the inside wall of the transfer line thereby simultaneously insuring line flexibility and minimizing the evaporation rate of the liquid drops.

The required two-phase flow regime in low wattage systems is maintained by heat absorption from the environment. At higher cooling loads and at higher flow rates, the heat absorbed from the environment is insufficient for the maintenance of the required two-phase flow regime. Under such conditions, the transfer tube wall is gradually cooled down until it reaches a critical temperature at which a liquid film is formed along the inside surface changing the flow regime to annular flow which is characterized by a high heat transfer coefiicient. This is followed by a step increase in heat absorption from the environment and the sudden loss of flexibility of the rubber transfer tube. The system becomes completely inoperative under such conditions, not only because of loss of tube flexibility, but also because the increased heat transfer along the tube results in excessive evaporation of the liquid with the resultant reduction of the cooling efliciency of the system to an impractical value.

The present invention extends the cooling capability of this type of cryogenic system at least one order of magnitude by the insertion of a high-intensity heater at the exhaust port of the supply dewar vessel. The necessary heater power is a function of the mass flow rate of the coolant.

An object of this invention is to maintain stable-twophase flow in uninsulated, fiexible-transfer-line cryogenic systems.

Another object is to maintain stable two-phase flow in the cooling load is in excess of approximately two watts.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic illustrating a cryogenic cooling system utilizing the present invention;

FIG. 2 is a curve showing required minimum heater exit qualities for a range of flow rates; and

FIG. 3 is a side view cross-section taken through the heater element block.

FIG. 1 illustrates a cryogenic cooling system which can be used to cool an IR vidicon camera tube. The system comprises a vacuum-insulated supply dewar 12 and a vacuum-insulated cell dewar 14 connected by means of a flexible transfer tube 16 which may be fabricated from silicone rubber tubing. The vessels are employed to hold liquid nitrogen. The tubing 18 at the exhaust port of the supply dewar 12 is encased by a heater element 20 consisting of a metallic block, such as copper, containing an electric heating element therein.

The cell dewar 12 (or load dewar vessel) may be employed to hold the cooling finger 22 of an infra-red vidicon camera tube which is to be cooled.

The remainder of the system comprises the vapor return line 26 and a suitable pressure-regulating means which may, for example, consist of an absolute pressure regulator valve 28 with a vapor discharge port, a pressure regulator 30, a pressure build-up coil 32, a solenoid valve 34, a relief valve 36, a manual relief valve 38, a check valve 40 and a fill fitting 42.

Different types of heater elements may be employed. One type consists of a commercial electrical heater As-inch in diameter and 1% inches long, rated at 40 watts for a 28 volt source, inserted inside a copper block which encloses the exhaust-port tubing of the supply dewar. The copper block is insulated with glass wool insulation to minimize heat loss to the environment.

A side view cross-section of the heater element block 44 is shown in FIG. 3. The block 44 consists of two halves which are screwed or bolted together. Two holes are bored through the block, one 46 to hold a cylindrical electrical heater and the other 48 to enclose the tubing from the exhaust port of the supply dewar.

Proper operation of any heater element requires that the heat flux (power per unit area) be sufliciently high to ensure film boiling in the heater section. Of course, a good thermal path is desirable to minimize heat losses.

Control of heater power as a function of mass flow rate is necessary in order to maintain the desired twophase flow regime. If sufficient heat is not provided, the flow regime, and consequently the system performance, becomes time dependent, the transfer tube temperature dropping with time until a critical temperature is reached, when a sudden drop in temperature occurs. With suflicient heater power, or with sufiiciently high heater exit quality, this breakdown does not occur but steady-state continuous operation is obtained. There exists a minimum heater power for a given flow rate that must be provided to prevent breakdown of the desired two-phase flow regime. These minimum heater powers expressed in terms of heater exit quality are indicated by the curve of FIG. 2. A system with the above-described heating element has been operated at liquid nitrogen flow rates of up to 5.5 lbs./hr. and at cooling loads estimated at 20 watts.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claim the invention may be uninsulated, flexible-transfer-line cryogenic systems where practiced otherwise than as specifically described.

3 4 We claim: and a pressure build-up coil disposed in said return A fiexible-transfer-tube cryogenic cooling system compath tubing intermediate said regulator valve and prising, in combination: said supply vessel;

an insulated upply vessel having an exhaust port and a solenoid valve disposed in said return path tubing tubing extending therefrom said vessel being adapted 5 intermediate said coil and said supply vessel; and to form a container for the coolant of said cryogenic a heater element enclosing said exhaust-port tubing of system, said tubing being fabricated from a material said supply vessel, said heater element being of sufhaving a high Coefficient of thermal ivi y; ficient capacity to provide the minimum heater exit an insulated load vessel; quality relative to the mass flow rate of the coolant a flexible transfer tube coupling said exhaust-port 10 required to keep the temperature of said flexible tubing to said load vessel; transfer tube above critical temperature. tubing connecting said load and supply vessels and constituting a return path for said coolant between r n s Cited Said vessels; f l th 1 t f UNITED STATES PATENTS pressure means or men a ing e coo an o sai sys- 15 2,997,855 8/1961 Templer 625l X tem at a predetermined mass rate of flow and for 3,126,711 3/1964 Miner 62 514 X regulating the pressure in the system; said pressure means including an absolute pressure regulator valve disposed in said return path tubing MEYER PERLIN Pnmary Examiner 

